Dielectric material



Dec. 15, 193s. BOOTH I 2,063,979

DIELECTRIC MATERIAL Filed llai'ch 23, 1935 WITNESSES: INVENTOR Patented Dec. 15, 1936 UNITED STATES PATENT OFFICE turing Company, East Pittsburgh, Pa, a corporation of Pennsylvania I Application 23,

This invention relates to dielectric liquids which are suitable to be employed as insulating mediums in electrical apparatus.

An object of this invention is to provide a dielectric medium which is comparatively stable and which has a high insulating value. I I

Another object of this invention is to provide a dielectric liquid for use with electrical apparatus which upon decomposition by electric arcs will produce a gas capableof quenching the arcs and flames encountered.

A further object of this invention is to provide a dielectric that is readily miscible with other dielectrics.

It has been discovered that benzotrifluoride and its halogenated derivatives are suitable to be employed as insulating fluids. These, liquids are comparatively stable, have a low power factor, a low freezing point and a high boiling point and are relatively non-viscous.

Reference is made to the accompanying drawing illustrating a particular embodiment of this invention, in which the single figure shows a transformer I having cables 2 and transformer elements 3 immersed in dielectric liquid 4 in,

accordance with this invention. Cable 2 outside the transformer l is also shown as being filled with the dielectric liquid 4.

It has been found by experimentation that benzotrifluoride may be employed as a dielectric liquid, but that the chlorinated derivatives of benzotrifluoride, either by themselves or in mixtures with each other, are preferred. In preparing the dielectric comprising the mixtures of the chlorinated benzotrifluorides, it is often advantageous' to carry the chlorination process to the extent that there can be only small amounts, if any, of monochloro substitution products present. These derivatives make a desirable insulating fluid which has a low power factor.

In preparing dielectrics, a series of halogenated derivatives of benzotrifluoride have been prepared, which is as follows: dichlorobenzotrifluoride, trichlorobenzotrifluoride, tetrachlorobenzotrifluoride, metafluorobenzotrifluoride. orthofluorobenzotrifluoride and parafluorobenzotrifluoride. It'is to be understood, however, that intermediate halogenated benzotrifluorides may also be prepared but that those set forth are the preferred dielectrics.

In.preparing the halogenated derivatives of benzotrifluoride, such as dichlorobenzotrifluoride and trichlorobenzotrifluoride, where it is desirable to employ. the halogenated derivatives rather than benzotrifiuoride which has been prepared 1935, Serial No. 12,748

and purified in the usual manner, the following method has been followed although other methods may be practiced.

The benzotrifluoride and a quantity of anhydrous ferric chloride are introduced into a suitable flask or container which is provided with a reflux condenser. The temperature of the contents of the container is maintained at to C. by means of a waterbath while a slow stream'of dried chlorine is lead into the mixture. Quantities of hydrogen chloride are thereupon evolved. This reaction slows down as the mixture gains weight but will increase when the temperature of the mixture is raised to C. Suchchlorination is continued until no reaction is noticed.

The chlorinated product is then distilled in a suitable manner and the distillate is agitated with concentrated hydrochloric acid to remove traces of ferric chloride and then carefully washed with saturated sodium carbonate solutlon and 20% sodium hydroxide solution followed by several treatments with distilled water. The mixture of dichlorobenzotrifiuorides and trichlorobenzotrlfluorides thus resulting is dried with calcium chloride. Upon fractionating the mixture, a clear colorless liquid results which has a boiling point of between and 220 C. and a freezing point below -15 C. A small amount of residue is found which is liquid at 10 C. and partially solidified at 15 C.

The method which may be followed in preparing the pure dichlorobenzotrifluoride (3,4-CcH'3C12CF3) is to start with p-toluidine and convert it into 3,4-dichlorotoluene by first pre- 35 paring p-acetotoluidide by boiling p-toluidine with glacial acetic acid in a flask surmounted by a reflux condenser, pouring the hot solution into an excess of cold water and filtering the-p-acetotoluidide from the acid solution, washing and 4 then drying it at 100 C. The p-acetotoluldlde is then dissolved in glacial acetic acid and concentrated hydrochloric acid and the resulting solution is cooled in ice. -A cold, saturated, aqueous solution containing sodium chlorate is 45 then added in small proportions after which a further addition of concentrated hydrochloric acid is made and the solution is boiled for a period of time under a reflux. The acid solution is distilled with steam to remove some of the 50 acid and oily impurities. When cooled and rendered alkaline with sodium carbonate and then further distilled 3-chloro-4-amino-toluene is obtained. The amino group is replaced by chlorine after which the side chain of the result- 55 ing compound is chlorinated without a catalyst and then fluorinated with resublimed antimony trifluoride and purified to produce the pure dichlorobehzotrifluoride (3,4-CeHsC1sCF3).

It has been found under test that the dichlorobenzotrifluorides (3,4-CeHaChCP'a) has a dielectric constant of 3.8 and a low power factor of 2, which with its high boiling point and low freezing point makes it desirable as an insulating fluid.

Other desirable insulatingfluids are obtained upon further chlorination of benzotrifluoride. Such chlorination results in mixtures of isomeric trichlorobenzotrifluoride (CeHsClsCFs) tetrachlorobenzotrifluoride (CsHChCFs) and small a m o u n t s o f pentachlorobenzotrifluoride (CaCltCFm). Such chlorination is obtained if pieces of iron gauze are placed in the chlorinating flask or container and the contents heated at 100 C. while passing in chlorine. As in the case where anhydrous ferric chloride is employed, chlorination stops after. a maximum boiling point of 220 C. is reached.

In order to introduce more chlorine into the benzene ring the contents of the flash or container is heated over a direct flame at such a temperature that crystals of ferric chloride will form but below the boiling point of the contents of the container while a stream of dry chlorine is lead into the mixture. After the chlorination ceases and the mixture is purified in the usual manner, it will be-iound that the mixture of isomeric trichiorobenzotrifluoride, :tetrachlorobenzotrifluoride and pentachlorobenzotrifluoride resulting, has a boiling point between 230 and 260 C. while crystals appear if the mixture is cooled to 15 C. Such a mixture has a low power factor of 2.8. I

Orthochlorobenzotrifluoride (O-CeHeClCFs). may be prepared by converting o-toluidine into orthochlorotoluene and chlorinating the orthochlorotoluene without a catalyst into o-chlorobenzotrichloride which, in turn, is fluorinated with an excess of sublimed antimony trifluoride. The resulting product is purified and dried to produce orthochlorob'enzotrifluoride, which has a boiling point of 152.8 C. and a freezing point of 'l.5 C.

Metachlorobenzotrifluorlde (m-CsHlClCFi) may be prepared by replacing the amino group of m-amino-benzotriiluoride with a chlorine atom. In order to effect the substitution, mamino-benzotrifluoride is mixed with concentrated hydrochloric acid and the amine is diasotized by the addition of a concentrated solution of sodium nitrite. The diazotised. solution is poured into a boiling solution of 10% cuprous chloride and the resulting mixture is distilled with steam and the water and the oil distillate extracted with ether; The solution is dried with anhydrous calcium chloride and fractionated. This product is agitated with a saturated solution of sodium carbonate, dried by refluxing with barium oxide and then distilled. The resulting product, metachlorobenzotrifluoride, has a boilings loint of 138.4 C., and a freezing point of C. I Parachlorobensotrifluoride (p-CoHaClCPa) may be produced by preparing parachlorotoluene, chlorinating the side chain to parachlorobenzotrichloride and replacing the chlorine atoms in the side chain with fluorine atoms by means of antimony trlfluoride. This halogenated deriva+ tive of benzotrifluoride is desirable as a dielectric material in thatit hasaboilingpoint of 139.3 C., a'

freezing point of 34 C. and a power factor of 4.8.

The halogenated derivativu of bensotrifiuoride which contain fluorine in both the side fluoride has a boiling point of 102.8' C., a freezing point of 41.7 C. and a power factor of 1.2.

Since it is difficult to control the substitution of the fluorine in the ring of benzotrifluoride, it is preferred to produce the halogenated derivatives such as parafluorobenzotrifluoride by starting with an easily controlled chemical such as para-toluidine. The amino group of para-toluidine is replaced by fluorine and the side chain is then successively chlorinated and fluorinated.

Metafluorobenzotrifluoride may be prepared direct from a base of benzotrifluoride by nitrating benzotrifluoride and reducing the nitro group to an amino group and then replacing the amino group by a fluorine atom.

Since any method of preparing complex organic compounds always results in the preparation of a large number of similar materials which are expensive to separate, it is preferred to employ the liquid mixtures as they are formed.

Benzotrifluoride and each of its halogenated derivatives described herein are suitable to be employed as insulating fluids. It has also been found that it is possible to chlorinate benzotrifluoride to form a liquid mixture which has a combined chlorine content such that the mixture will have no flash or fire point. Further. these halogenated derivatives are such good solvents that they will mix readily with other halolenated liquids to produce a mixture which has an extremely low pour point. It has been found advantageous to mix these halogenated derivatives with other stable halogenated products such as the chlorinated cyclic compounds consisting of chlorinated diphenyl, diphenyl ethers and naphthalene, as well as the somewhat less stable chlorinated benzenes andtoluenes.

This invention has been described with reference to particular methods of producing the different dielectric liquids. but it is to be understood that the invention is not to be limited by such description for within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim as my invention:

1. A dielectric liquid comprising a mixture of the halogenated derivatives of bensotriflnoride comprising isomeric trichlorobensotrifluorides. tetrachlorobensotrifluorides. and pentachlorobenzotrifluoride.

2.Adielectricliquideomprisingamixtm'eof halogenated derivatives of bensotriflmride comprising isomeric trichlorobensotrifiuoridel. tetrachlorobenzotrifluorides and pentachlorobenaotrifluoride with one or more compounds from the chlorinated cyclic group consisting of chlorinated diphenyl. diphenyl ether and naphthalene.

3. A dielectric liquid comprising a mixture of halogenated derivatives of bensotriflnorlde comprising isomeric trichlorobensotriflnoridel. tetra chlorobenzotrifluorides and pentachlorobenlotrifiuoride with chlorinated diphenyl.

EAROLDEBOOTB. 

